Mtn took over the contracts from Nashua Mobile

First of all Mtn took over the cellular contracts of Nashua mobile without us even knowing about it. My account went from+ -R700.00/pm at Nashua to R1700.00/pm at Mtn and I have tried to speak to someone at Mtn and after several calls,and holding on for at least 40 minutes at a time, I always get a different response and empty promises that someone will contact me and that is where it stays. It is so sad as I never in 15 yrs had any problems with Nashua. I am now at the stage where I want to get out of this contract as I signed it with Nashua and not Mtn. How do I get someone senior to talk to at Mtn that will sort this mess out. I am soooo frustrated you can not believe it!!!!!